I would suggest using the following layout, as I dont think that there are that many buyers for each of the country specific layout.

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of course the ö and ä keys would only have to contain 2 letters (Æ / Ä) and (Ø / Ö) as we can just interchange them.

I'm a Swede, and only have ISO keyboards, and I hate combined nordic layouts. I'd rather have a Norwegian or Danish (or German) set with one set of legends on the alphas, so while I would love a Swedish set, I won't buy a nordic kit.

That's the Irish set Ivan did and I plan on doing as part of the Royal Alpha inspired set.

Nice! I would prefer the look of the high contrast black/blue/red legends (perfect for dye sub) in the "US int" layout compared to the low contrast light green pad print legends. I'm sure both would look great though.

I think ISO is more important than G80-1800 addons, as there are more european users than G80-1800 users.

But how many keys does it take to make it compatible for all ISO users?

I think most ISO users would be OK with a 4-key kit: Enter, Shift, and two 1u keys (R1 & R2), all of them necessary to fill out an ISO keyboard. Only half of those keys are included. I'd rather have all 4 or none (and a later custom 4 key kit).